Merge pull request 'dline: fix odd x1/x2 handling with C_INVERT color (render-cg)' (#39) from Yatis/gint:fix/dline_fxgc into dev

Reviewed-on: https://git.planet-casio.com/Lephenixnoir/gint/pulls/39

src/render-cg/gint_dline.c

@@ -15,12 +15,17 @@ void gint_dhline(int x1, int x2, int y, int color)
 	int offset = DWIDTH * y;
 
 	/* Use longwords to do the copy, but first paint the endpoints to heed
-	   for odd x1 and x2. Checking the parity may be a waste of time. */
+	   for odd x1 and x2. Checking the parity may be a waste of time for
+	   "real" color, but must be checked when C_INVERT in involved to
+	   avoid "cancelling" invert effect with potential overdraw of the
+	   next operation. */
 	if (color != C_INVERT) {
 		gint_vram[offset + x1] = color;
 		gint_vram[offset + x2] = color;
 	} else {
+		if (x1 & 1)
 			gint_vram[offset + x1] ^= 0xffff;
+		if (x2 & 1)
 			gint_vram[offset + x2] ^= 0xffff;
 	}